Well, the first solution is dependent on the height of the left column being fixed, which it will not be. The second solution is dependent on the center two columns being taller than the left, which is also not guaranteed. But thanks for trying. (Note - the heights given in the original example are for testing. In production height should be set to 'auto.'

Thanks for looking into this. I finally fixed it through a different route primarily because I was asked to change how the layout behaves on a smaller resolution set. I ended up with the left two columns being fixed with the rightmost alone being flex, which is much, much easier to work with.